1. FIELD OF THE INVENTION
The present invention relates to an optical transmitter for generating optical signals to be transmitted through transmission media such as optical fibers having chromatic dispersion that can degrade transmission quality, and a method for controlling such an optical transmitter.
2. DESCRIPTION OF THE BACKGROUND ART
In the optical transmission system, the degradation of transmission quality caused by the waveform distortion due to chromatic dispersion of the optical fibers that are used as transmission media is of great concern. This phenomenon occurs when an optical pulse width is increased to cause interferences with respect to neighboring time-slots as a bandwidth of optical signals is affected by the group velocity dispersion of the optical fibers.
FIG. 1 shows a prior art as disclosed in Japanese Patent Application Laid Open No. 10-79705 (1998). This prior art reference proposes an optical modulation apparatus in which the group velocity dispersion is cancelled by applying a pre-chirp to optical signals at a transmitter, so as to suppress the degradation due to the group velocity dispersion. In this proposition, the waveform distortion at a receiving end is suppressed by applying a frequency chirp in an amount that nearly matches the group velocity dispersion of the optical fibers at an optical transmitter in advance.
In FIG. 1, the continuous-wave light generated by a light source 101 is divided into two optical clocks by a clock generator 102. This clock generator 102 can be a Mach-Zehnder optical modulator, for example, which is to be driven by, sinusoidal wave electric signals. At this point, a value and a sign of the chirping are controlled by controlling a bias voltage to be applied to the optical modulator or the like at the pre-chirp unit 106. Also, in the case of using an electro-absorption optical modulator as the clock generator 102, the amount of the chirping is adjusted by controlling the bias voltage to be applied to the electro-absorption optical modulator at the pre-chirp unit 106. Then, the chirp controlled optical clocks are encoded by first and second data modulators 103 and 104 provided as next stage optical modulators, and then multiplexed by an optical multiplexer 105 to obtain desired optical signals.
However, in this optical modulation apparatus, the frequency chirp in an amount for cancelling the group velocity dispersion is applied in advance so that a bandwidth of the optical signal spectrum is increased. The optical spectrum bandwidth is inversely proportional to the square of the tolerance with respect to the group velocity dispersion, so that the tolerance is progressively lowered when the bandwidth is increased. Consequently, this conventional scheme for applying the pre-chirp lowers the dispersion tolerance and thereby hampers a stable operation of a transmission system. In other words, the transmission quality will be degraded even by a slight difference in the group velocity dispersion.
Also, in the case of using the electro-absorption optical modulator as the clock generator 102, the non-linear dependency of the extinction ratio with respect to the bias voltage as shown in this prior art reference can cause a problem. Namely, when the bias voltage is changed in order to control the frequency chirp, the clock pulse width is also varied so that the optical spectrum bandwidth is also changed and the dispersion tolerance is also changed. These chirp and pulse width cannot be varied independently, so that it is also difficult to set a desired amount of the chirping stably.
In addition, there is a large optical power loss at a time of extracting clock light from continuous-wave light using a gate provided by a modulator in the clock generator 102, and the loss is further increased as two encoded clock lights extinguish each other due to the optical interference effect at a time of multiplexing at the optical multiplexer 105. This causes a decrease in the optical power at an output end of the optical modulation apparatus, which in turn causes a lowering of S/N ratio at a time of transmission.
FIG. 2 shows three graphs indicating a relationship between the dispersion tolerance and the chromatic dispersion of optical clock having a bit rate of 80 Gbit/s, using a relative optical phase of two encoded optical clocks at a time of multiplexing at an optical multiplexer as a parameter. When the bit rate is different, the absolute value of dispersion (|D|(ps/nm)) indicated on the horizontal axis is changed but a relative relationship regarding an amount of penalty with respect to the duty ratio and the relative optical phase difference remains the same. Here, xe2x80x9cin-phasexe2x80x9d is the case where the relative optical phase difference is 0 or integer multiple of 2xcfx80, xe2x80x9cout-of-phasexe2x80x9d is the case where the relative optical phase difference is an odd integer multiple of xcfx80, and xe2x80x9cmiddle-phasexe2x80x9d is the case where the relative optical phase difference is xcfx80/2 or odd integer multiple of xcfx80/2.
As shown in FIG. 2, the degradation of receiver sensitivity becomes noticeable when the duty ratio is excessively increased. In other words, in order to realize both a high dispersion tolerance and a low receiver sensitivity degradation, there is a need to control the duty ratio optimally. However, in the prior art, the optical clock is generated using sinusoidal waves in a frequency equal to the bit rate before the multiplexing by setting the driving point at a linear portion of the modulator, and controlling this optical clock generation at a desired value will require the change of modulation level, which in turn will cause a problem of the extinction ratio degradation. Moreover, the signal waveform will be degraded due to the interference effect at a time of the multiplexing.
Also, Japanese Patent Application Laid Open No. 10-79705 (1998) discloses an exemplary case of driving by rectangular waves, but such a driving by rectangular waves widens the optical spectrum excessively so that the dispersion tolerance will be lowered. For this reason, it is expected that the influence of the group velocity dispersion of the optical fibers with respect to the transmission distance becomes more stringent. Also, a driving by sinusoidal waves can only realize excessively broad duty ratio, so that in the case where the number of divisions is 3 or more as shown in this prior art reference, it is difficult to generate practically effective optical signals because the interference effect between adjacent optical clocks is so large that they extinguish each other.
On the other hand, FIG. 3 shows another prior art as disclosed in Japanese Patent Application Laid Open No. 9-261207 (1997), in which outputs of optical modulators 111 and 112 are multiplexed at an optical multiplexer 113 while a low frequency signal generated by a low frequency oscillator 115 are superposed by a phase modulation to the optical signal in one of them, and a part of the optical signal after the multiplexing is monitored by an optical phase detection and control unit 116 and the phase control is carried out by an optical phase control unit 110 such that the intensity of the intensity modulated component of the low frequency signal in the monitored part becomes minimum, so as to automatically maintain the relative optical phase difference.
However, the minimum value control has a rather poor sensitivity so that it is inevitable to superpose the low frequency, signal at relatively large amplitude as already shown in this prior art reference. However, as already mentioned above, this amounts to displacing the relative optical phase difference from xcfx80 intentionally so that it approaches to the case of the relative optical phase difference equal to xcfx80/2 which is associated with the severe degradation of dispersion tolerance. In other words, this controlling itself causes the degradation of dispersion tolerance.
As described, in the conventional optical transmitter that attempts to suppress the degradation of transmission quality due to the group velocity dispersion by controlling the chirp applied to the optical clock at a time of generating the optical clock using sinusoidal waves or the like, there is a problem in that the excessively wide optical spectrum bandwidth will be occupied as the chirp is applied, so that the tolerance with respect to the group velocity dispersion becomes lower.
Also, even though the chirp can be controlled to be arbitrarily small by the push-pull driving of a push-pull type Mach-zehnder optical modulator, there is a problem that the duty ratio of the optical clock cannot be controlled.
In addition, when the electro-absorption optical modulator is used, there is a problem that it is difficult to apply a desired amount of the chirping because the amount of the chirping itself is large and the chirping parameter and the duty ratio are not independent. Moreover, there is also a problem that controlling the duty ratio arbitrarily will cause a large variation in the loss, so that a variation in the S/N ratio at a time of communication will be caused and a fluctuation in the transmission quality will be caused.
It is therefore an object of the present invention to provide an optical transmitter and an optical transmitter control method capable of realizing a high tolerance with respect to the group velocity dispersion of the optical fibers, a small receiver sensitivity degradation, and an improved stability that is hardly affected by the group velocity dispersion even in the case of network scale expansion.
According to one aspect of the present invention there is provided an optical transmitter, comprising: a light source section configured to generate optical clock pulses synchronized with a signal bit rate while maintaining a duty ratio of the optical clock pulses constant, which is capable of variably setting the duty ratio; and an encoding section configured to encode the optical clock pulses by using electric signals synchronized with the optical clock pulses while setting a relative optical phase difference between the optical clock pulses in adjacent time-slots to be an odd integer multiple of xcfx80.
According to another aspect of the present invention there is provided an optical transmission apparatus, comprising: a plurality of optical transmitters provided in parallel and set to output optical signals of mutually different optical wavelengths, each optical transmitter having a light source section configured to generate optical clock pulses synchronized with a signal bit rate while maintaining a duty ratio of the optical clock pulses constant, which is capable of variably setting the duty ratio, and an encoding section configured to encode the optical clock pulses by using electric signals synchronized with the optical clock pulses while setting a relative optical phase difference between the optical clock pulses in adjacent time-slots to be an odd integer multiple of xcfx80; and a wavelength division multiplexer configured to output signals obtained by wavelength division multiplexing the optical signals of mutually different optical wavelengths outputted from the plurality of optical transmitters.
According to another aspect of the present invention there is provided an optical transmitter control method, comprising the steps of: variably setting a duty ratio of optical clock pulses to a value that makes interferences between pulses small, using a configuration capable of variably setting the duty ratio; generating the optical clock pulses synchronized with a signal bit rate by maintaining the duty ratio constant; and encoding the optical clock pulses by using electric signals synchronized with the optical clock pulses while setting a relative optical phase difference between the optical clock pulses in adjacent time-slots to be an odd integer multiple of xcfx80.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.